Method and circuit for trimming the frequency of an oscillator

ABSTRACT

An integrated circuit oscillator which includes a capacitor, a reference current source coupled to the capacitor used to charge the latter, and a trigger circuit coupled to the capacitor having an upper input threshold for changing from a first state to a second state and a lower input threshold for changing from the second to the first state. A discharge circuit is coupled to the trigger circuit and is operative to discharge the capacitor in response to the trigger circuit changing states and to cease the discharging on changing back to its original state.

BACKGROUND OF THE INVENTION

The present invention relates to a method and circuit for use intrimming the frequency of an oscillator which is particularly adaptedfor use in VLSI technology.

In fabricating oscillators on VLSI chips it is standard to fabricate aresistor and capacitor which have, in combination, an RC time constantthat determines the frequency of the oscillator. Because of differencespresent in the fabrication process of the chips it is usually necessaryto provide for some means of trimming the oscillation frequency of theoscillator. A standard approach for trimming the frequency of anoscillator is to somehow change the value of the capacitor or theresistor through which the capacitor is charged. Such a technique,however, is quite imprecise because of the parasitic impedance of theswitching devices.

Accordingly, it is an object of the present invention to provide animproved circuit for trimming the frequency of oscillation of anoscillator. It is a further object to provide a circuit in which thefrequency of oscillation of an oscillator can be changed more accuratelythan hitherto known circuits.

SUMMARY OF THE INVENTION

According to the invention there is provided an integrated circuitoscillator which includes a capacitor, a reference current sourcecoupled to the capacitor for charging the latter and a trigger circuitcoupled to the capacitor. The trigger circuit has an upper inputthreshold for changing from a first to a second state and a lower inputthreshold for changing from the second to first state. A discharge meansis coupled to the trigger circuit and is operative to discharge thecapacitor in response to the trigger circuit changing from the firststate to the second state and to cease discharging the capacitor inresponse to changing from the second state to the first state.

Preferably, the oscillator includes a mirror current source coupled tothe reference current source which is operative to pass a current equalto a constant factor of the reference current and a switch in serieswith the mirror current source operative to become conductive inresponse to a control signal applied thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asother features and advantages thereof, will be best understood byreference to the detailed description which follows, read in conjunctionwith the accompanying drawings, wherein:

FIG. 1 is a circuit diagram of an oscillator whose frequency is trimmedby inserting resistors in parallel to charge a capacitor;

FIG. 2 is a circuit diagram of an oscillator whose frequency is trimmedor changed by inserting in parallel mirrored current sources; and

FIG. 3 is a circuit in which mirrored current sources are used not onlyto charge but also to discharge a capacitor.

DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS

Referring to FIG. 1 there is shown a standard oscillator used inintegrated circuits which consists of a primary resistor R coupledbetween the high voltage source V_(cc) and a capacitor 10 coupled toground. A current I_(R) which is generated is used to charge up thecapacitor 10. As a result the voltage on line 44 rises with a timeconstant of RC where C is the value of capacitance of capacitor 10. Line44 is connected to the input of a Schmitt trigger circuit 34. The outputof the Schmitt trigger circuit is fed to an inverter 38 which in turndrives the gate of an N-channel transistor 42 coupled across capacitor10. The output of the inverter 38 is taken along line 40 to provide theoscillator output. When the voltage on line 44 reaches an upper inputthreshold of the Schmitt trigger circuit 34 the latter changes from afirst state in which its output is high to one in which its output islow. Thus, the output of inverter 38 changes from low to high inresponse to the change in state of the Schmitt trigger circuit 34 andturns on transistor 42 while transistor 23 in series with capacitor 10is turned off by the low output of Schmitt trigger 34 on line 36. Oncethe voltage on line 44 fails below a lower input threshold of theSchmitt trigger due to the discharging of capacitor 10 by transistor 42,the Schmitt trigger circuit 34 changes back from the second state to thefirst causing the output on line 40 to fall and transistor 42 to ceaseconducting.

To trim the oscillator frequency in order to adjust for processdifferences in its fabrication from one chip to the next a series ofresistors R₁, R₂, . . . R_(N) are placed in parallel with resistor R butwith associated switches formed by transistors 20, 22, . . . , and 24,respectively, in series with the aforementioned resistors. By designingR to be too high, it is then possible to turn on any combination ofswitches 20, 22, . . . , and 24 to insert a corresponding combination ofresistors R₁, R₂, . . . , and R_(N) in parallel with resistor R and toincrease the frequency of oscillation accordingly. Transistors 20, 22, .. . , and 24 are turned on by a binary code converter 26 whose outputspass through inverters 28, 30, . . . , and 32 to the gates oftransistors 20, 22, . . . , and 24, respectively.

An alternative approach to trimming resistor values is to trim capacitorvalues by placing a number of capacitors in parallel with capacitor 10.However, both of these methods are imprecise due to the parasiticimpedance of the switching devices. In order to avoid the latterdependence the circuit of FIG. 2 utilizes current generators to chargecapacitor 10.

A reference current I_(R) is developed by a circuit consisting of aconventional current source 27 coupled between Vcc and drain oftransistor 25. The drain of transistor 25 is coupled to its gate whileits source is connected to ground. A transistor 21 whose gate isconnected to the gate of transistor 25 has its drain connected to thesource of P-channel transistor 11 and its source connected to ground.The gate of P-channel transistor 11 is connected to its source. Thus,current source 27 and transistor 25 establish a gate to source voltagefor transistor 21 equal to the gate-to-source voltage of transistor 25and hence a current I equal to the current through transistor 25.

Since the gates of transistors 12, 14, 16, . . . , and 18 are connectedto the gate of transistor 11, and their drains to Vcc, the currentthrough each of the latter transistors is equal to the ratio ofwidth/length for the transistor to the W/L of transistor 11. Thus, thecurrent through transistor 12 is equal to that through transistor 11,that for transistor 14 equals W1/W that of transistor 11, that fortransistor 16 equals W2/W that of transistor 11, etc. where the lengthof each transistor is the same. The gate control signals for transistors20, 22, . . . , and 24 are the same as those shown in FIG. 1 in whichlike parts are designated by the same reference numbers as those shownin FIG. 2. Thus, it is possible to set I₁, I₂, . . . , and I_(N) bysetting the values of W₁, W₂, . . . , and W_(N), respectively, using aconstant L for each transistor 12, 14, 16, . . . , and 18.

It is obvious that by using mirrored current sources in this way it isstraight forward to obtain weighted ratios for the currents through eachtransistor 14, 16, . . . , and 18. One could for example establish abinary set of ratios by making I₁ =2I_(R), I₂ =4I_(R), . . . , I_(N)=2^(N) I_(R). One could also use two current mirrors to produce atriangular waveform by the circuit shown in FIG. 3.

In this case two mirrored current sources 138 and 142 are coupledbetween V_(cc) and line 144 and ground and line 144, respectively,through transistor switches 140 and 136, respectively. Transistor switch140 has its gate connected to the output 148 of Schmitt trigger 134while that of transistor 136 is connected to the output 146 of inverter132. Thus, when the voltage on line 144 is below the upper inputthreshold of Schmitt trigger 134, the output on line 148 is high andtransistor 140 is conducting current from current source 138 intocapacitor 100. Once the voltage on line 144 reaches the upper inputthreshold of trigger 134, the latter changes state cutting offtransistor 140 and turning on transistor 136. Now current source 142conducts a fixed current from capacitor 100 to discharge the latterlinearly so that the combined waveform on line 144 is a sawtoothdepending only on the capacitance and the current values conducted bycurrent sources 138 and 142. Although the mirrored current sources havebeen indicated only by the double circles, in fact they would be as inFIG. 2 with a binary translater 26 as in FIG. 2 driving the switchtransistors 140 and 136 from current sources 138 and 142, respectively.

While this invention has been described with reference to anillustrative embodiment, this description is not intended to beconstrued in a limiting sense. Various modifications of the illustrativeembodiment, as well as other embodiments of the invention, will beapparent to persons skilled in the art upon reference to thisdescription. It is therefore, contemplated that the appended claims willcover any such modifications or embodiments as fall within the truescope of the invention.

What is claimed is:
 1. An integrated circuit oscillator, comprising:acapacitor; a reference current source for providing a reference current;a mirror current source coupled to said reference current source and tosaid capacitor operative to charge said capacitor with a current equalto a predetermined factor of the reference current of said referencecurrent source; a trigger circuit coupled to said capacitor having anupper input threshold for changing from a first state to a second stateand a lower input threshold for changing from the second to the firststate; discharge means coupled to said trigger circuit operative todischarge said capacitor in response to said trigger circuit changingfrom the second state to the first state.
 2. An oscillator according toclaim 1, wherein said trigger circuit is a Schmitt trigger circuit andsaid discharge means is a transistor having a source to drain pathcoupled across said capacitor and a gate coupled to an output of saidSchmitt trigger circuit.
 3. An oscillator according to claim 2,including a capacitor switch in series with said capacitor operative toopen and shut off charging current to said capacitor in response to saidtrigger circuit changing from the second to the first state.
 4. Anoscillator according to claim 1, wherein said reference current sourceis a P-channel transistor whose gate is coupled to its source, itssource to a constant current and its drain to a high voltage sourceV_(cc).
 5. An integrated circuit oscillator, comprising:a capacitor; areference current source for providing a predetermined current; atrigger circuit coupled to said capacitor having an upper inputthreshold for changing from a first state to a second state and a lowerinput threshold for changing from the second to the first state;discharge means coupled to said trigger circuit operative to dischargesaid capacitor in response to said trigger circuit changing from thefirst state to the second state and to cease discharging said capacitorin response to changing from the second state to the first state; amirror current source coupled to said reference current source and tosaid capacitor and operative to pass a current equal to a predeterminedfactor of said reference current; and a switch in series with saidmirror current source operative to become conductive in response to acontrol signal applied thereto.
 6. An oscillator according to claim 5,wherein said trigger circuit is a Schmitt trigger circuit.
 7. Anoscillator according to claim 5, wherein said discharge means is atransistor connected across said capacitor operative to turn on inresponse to a binary translator control signal applied to its gate. 8.An oscillator according to claim 5, wherein said reference currentsource is a P-channel reference transistor having a drain connected tohigh voltage source V_(cc), a gate connected to its source and itssource coupled to a constant current source.
 9. An oscillator accordingto claim 8, wherein mirror current source is a P-channel transistorwhose gate is coupled to the gate of said reference transistor, whosedrain is connected to V_(cc) and whose source is connected to saidswitch.
 10. An oscillator according to claim 9, wherein said switch isan N-channel transistor having a source to drain path coupled betweenthe source of said mirror current transistor and said capacitor.
 11. Anoscillator according to claim 5, wherein said discharge means is adischarge current source in series with a discharge switch coupledacross said capacitor and operative to become conductive in response tosaid trigger circuit changing from the first state to the second state.12. An oscillator according to claim 6, wherein said Schmitt trigger isinverting and an output of the latter is coupled to an inverter with theinverter output coupled to a gate of discharge transistor whichdischarges said capacitor on becoming conductive.
 13. A method ofcontrolling the frequency of an oscillator, comprising:charging acapacitor with a primary current source; applying the voltage to whichsaid capacitor is being charged to an input of a Schmitt trigger circuithaving an upper input threshold and a lower input threshold andoperative to change from a first state to a second state in response tothe voltage at its input exceeding said upper input threshold and tochange from the second state to the first state in response to thevoltage at its input falling below said lower input threshold; chargingsaid capacitor with a mirror current source connected in parallel withsaid primary current source, said mirror current source providing acurrent of a selectable value, so that the time required to charge saidcapacitor to the voltage of said upper input threshold is adjusted; andapplying the output of said Schmitt trigger to a discharge transistorcoupled across said capacitor so as to become conductive upon saidSchmitt trigger switching from said first to said second state.